1. Field of the Invention
The present invention relates to a moving object image pickup apparatus for detecting the position of a moving object, such as an automobile, running in an image pickup region set on a road surface, and clearly picking up an image of the moving object.
2. Description of the Related Art
The following is a known example of a position detecting device of the aforementioned moving object image pickup apparatus. An ITV (industrial TV) camera, serving as image pickup means, continuously picks up a plurality of images in an image pickup region including a moving object and a fixed object. The images are stored in an image memory. The contents of the image memory are analyzed to calculate a position of the moving object, which is displayed on a screen of a monitor.
For example, FIG. 1 shows an example of an image picked up by an ITV camera. In this case, as shown in FIG. 1, the positional relationship between a fixed object 12 (e.g., a white line on a road surface 13, serving as a reference) and a moving object 11 (e.g., a tire of an aircraft or an automobile) is to be determined. FIG. 2 shows an example of a luminance distribution of the picked-up image. In FIG. 2, reference numerals 11a, 12a and 13a respectively denote luminance distributions of the tire 11, the white line 12 and the road surface 13.
In general, the luminance of the fixed object, i.e., the white line 12, of the picked-up image is greater than that of the road surface 13. Since a suitable threshold (threshold 2) can therefore be calculated from the luminance distribution shown in FIG. 2, the white line 12 can be extracted by converting the image shown in FIG. 1 to binary values using the threshold 2. The extracted image of the white line is projected to X and Y coordinates, and the position of the fixed object (white line 12) in the image shown in FIG. 1 can be obtained on the basis of the projected data.
The luminance of the moving object, the tire 11, is less than that of the other portions. Therefore, the image of the tire 11 can be extracted by converting the image shown in FIG. 1 to binary values using a suitable threshold (threshold 1) calculated from the luminance distribution shown in FIG. 2. The extracted image of the tire 11 is projected to X and Y coordinates, and the position of the moving object (tire 11) can be obtained on the basis of the projected data.
The positions obtained in the manner as described above can be geometrically converted to values on an actual road surface from a position and an angle of the ITV camera and an angle of field, thereby calculating the distance between the moving object and the fixed object.
The conventional moving object image pickup apparatus as described above has a following drawback. If the luminance of the image pickup environment increases, the difference in luminance between a relatively bright object and the other portion is reduced. As a result, the brighter object cannot easily be extracted. On the other hand, if the luminance of the image pickup environment decreases, the difference in luminance between a relatively dark object and the other portion is reduced. As a result, the darker object cannot easily be extracted.
The following is a known example of an apparatus, only for picking up a clear image of a moving object, of the moving object image pickup apparatus. An ITV camera picks up an image including a moving object (automobile) in an image pickup region set on a road surface. The image is stored in an image memory. The content of the image memory is analyzed to generate a lens control signal. The image pickup lens of the ITV camera is controlled by the lens control signal, so that an optimal image can be obtained. The control of the image pick-up lens means controls the enlargement ratio, the aperture, the focal length, etc.
For example, when the enlargement ratio is to be controlled, the picked-up image is analyzed and the enlargement ratio of the image pickup lens is controlled so that an image of the target object (automobile) can be picked up in a size (target value) preset by the operator. More specifically, the image of the moving object is cut off from the overall picked-up image, and if the size of the image of the moving object is greater than the preset size, the enlargement ratio is lowered and an image is picked up and analyzed again. On the other hand, if the size of the image of the moving object of the picked-up image is smaller than the preset size, the enlargement ratio is increased and an image is picked up and analyzed again. This operation is repeated until the size of the image is optimized.
When the aperture is to be controlled, the luminance histogram of the picked-up image is analyzed and the aperture of the image pickup lens is controlled so that the luminance distribution can be a preset optimal distribution (target value). More specifically, if a greater number of pixels are distributed in a high luminance portion of the luminance histogram, the image pickup lens is controlled so as to narrow the aperture, and an image is picked up and analyzed again. If a greater number of pixels are distributed in a low luminance portion of the luminance histogram, the image pickup lens is controlled so as to widen the aperture, and an image is picked up and analyzed again. This operation is repeated until the distribution is optimized.
When the focal length is to be controlled, the frequency distribution of the picked-up image is obtained, and the focal length of the image pickup lens is controlled so that the frequency distribution can be a preset optimal distribution (target value). More specifically, the frequency distribution of the picked-up image is analyzed, while the focal length of the image pickup lens is being changed. The operation is repeated until an image having a component of the highest frequency is obtained.
The above conventional image pickup apparatus as described above, however, is disadvantageous in that the target values such as the enlargement ratio, the aperture, and the focal length must be preset by the operator in accordance with the image region. In addition, when controlling the aperture, it takes a considerable time for the aperture to be changed to a preset value after an aperture control signal is generated. Particularly, in a case of feedback control, the control of the aperture cannot overtake the high-speed movement of the moving object, and an optimal image cannot be obtained.